This invention relates to metal hydride electrodes and electrochemical cells incorporating the same.
Sealed or semi-sealed nickel metal hydride alkaline cells employ hydrogen storage alloy materials as the electrochemically active material for the negative electrode. These alloy materials are believed to possess catalytically active sites for dissociating hydrogen molecules to increase the rate at which hydrogen absorption and desorption can be accomplished. Cells made with these hydrogen storage alloy materials forming the basis of the negative electrode are typically matched against nickel positive electrodes with alkaline electrolyte in a sealed configuration operating in some respects analogously to a nickel-cadmium cell.
In a nickel metal hydride cell oxygen is generated at the positive nickel electrode during charge and overcharge and, particularly in starved electrolyte systems, passes directly through the porous, unsaturated separator layer to attack and oxidize the hydrogen storage alloy material in the negative plate. The metal hydride alloy, once oxidized, no longer serves as an efficient hydriding material since the resultant insulative oxide layer that is formed on the alloy passivates the alloy and prevents it from absorbing and desorbing hydrogen, particularly at elevated rates of discharge and charge. Thus oxygen and hydrogen can accumulate in the cell at concentrations sufficient to form an explosive mixture, provided an ignition source is supplied. In addition, some forms of the oxidized alloys are soluble in the alkaline electrolyte and become dissolved and migrate away from the negative electrode. Furthermore, oxidized hydrogen storage alloy materials lose conductivity. These problems stemming from oxidation of the alloy are not present in the well-known nickel-cadmium sealed cell wherein the cadmium oxide which is formed readily hydrolyzes to cadmium hydroxide, an electrochemically active form of cadmium.
In nickel-cadmium sealed cells it is known that cell pressure can be reduced by coating the cadmium electrode with carbon (see, for instance, U.S. Pat. No. 4,614,696 to Ito et al. and U.S. Pat. No. 4,987,041 to Simon and Japanese patent application no. 58-190444).
A primary object of this invention is to provide means for protecting the hydrogen storage alloy material in the negative electrode to prevent it from becoming oxidized during operation of a cell incorporating the electrode, and thereby maintaining the hydriding capability of the alloy, prevent its dissolution in the electrolyte, and maintain high levels of conductivity.